The fourth generation, 4G, wireless access within the 3rd generation partnership project, 3GPP, long-term evolution, LTE, is based on orthogonal frequency-division multiplexing, OFDM, in downlink and discrete Fourier transform, DFT, spread OFDM, also known as single-carrier frequency-division multiple access, SC-FDMA, in uplink, UL. Here, the UL physical channels consist of the physical uplink shared channel, PUSCH, the physical uplink control channel, PUCCH, and the physical random-access channel, PRACH, as well as of physical signals referred to as the demodulation reference signal, DRS, and the sounding reference signal, SRS.
In UL, PRACH is used for initial access by wireless devices wishing to access the communication system. The PRACH is also used for estimating timing offset between a wireless device and the base station, eNB, or network node, which receives the PRACH. A description of this procedure is given in 3GPP TS 36.213, V11.3.0. Upon reception in, e.g., the eNB, the PRACH must thus be detected with high accuracy and accurate timing offset estimation must be done.
With reference to FIG. 2, when a wireless device uses the PRACH, it transmits a so-called random-access preamble sequence in a known time/frequency resource 104 in the OFDM grid 105.
An illustration of PRACH signaling, as specified for LTE, see, e.g., 3GPP TS 36.211, V11.3.0, is shown in FIG. 2. Here four different formats, shown in FIG. 2 as format 0 through format 3, are specified where a PRACH preamble consists of one or two preamble sequences, each of length 24 576 samples. These preambles have a cyclic prefix, CP, of length between 3 168 and 21 024 samples for format 0 to 3. A fifth format, shown as format 4 in FIG. 2, is specified for time-division duplex, TDD, systems.
A long preamble sequence potentially carries more signal energy than a shorter sequence. Long preamble sequences can therefore in some cases be easier to detect when received with noise, since increased received signal energy allows for increased detector sensitivity. However, in general, the longer the preamble sequence, the larger is also the detection delay, as well as the sensitivity to the coherence time of the radio-propagation channel over which the preamble sequence has propagated. A problem then, is how to balance detector sensitivity with detection delay and sensitivity to channel coherence time in a random-access mechanism, such as the one used in the PRACH.
The illustration in FIG. 2 shows five different types of preamble sequence. A receiver configured to detect all allowable preamble sequences in parallel usually implements separate detectors for each type of preamble sequence. Therefore, in general, the larger the number of different preamble sequences is, the higher the preamble receiver complexity and processing requirements become. Thus, increasing the number of preamble sequences in a communication system drives receiver complexity and cost. A further problem then, is how to allow for an increased number and wider variety of preamble sequences in a communication system, while keeping processing requirements in the preamble sequence receiver reasonable.
The contribution R1-140743 to 3GPP TSG RAN WG1 Meeting #76 discusses random access using PRACH, and in particular a method in which a pre-determined preamble sequence is repeated in order to increase cell coverage. R1-140743 does not provide any solutions to the problems discussed herein.
Thus, there is a need for an improved PRACH signaling technique which balances detector sensitivity, detection delay, and sensitivity to channel coherence time in a random-access mechanism such as the one used in the PRACH while still keeping receiver processing requirements and complexity reasonable. It is an object of the present disclosure to provide solutions to, or at least mitigate, the above-mentioned deficiencies in the art.